Nuon Chea, the chief lieutenant of Khmer Rouge leader Pol Pot, and Khieu Samphan, who acted as head of state for the Maoist regime, are currently on trial for genocide and crimes against humanity.

They were responding to questions posed by the so-called civil parties, who are representing the victims' families at the United Nations-backed trial.

"As a leader, I must take responsibility for the damage, the danger to my nation," Nuon Chea said, offering "deepest condolences" to relatives of those who died.

"I feel remorseful for the crimes that were committed intentionally or unintentionally, whether or not I had known about it or not known about it," he said.

Khieu Samphan told Yim Roum Doul, whose father died during Khmer Rouge rule, that he didn't know at the time of "the atrocities committed by the military commanders and leaders."

"I feel extremely sorry for the disappearance and extremely brutal killing of your father," he said.

"I did not know the great suffering of our people," he said, adding that the perpetrators "must be brought to justice."

Pol Pot was able to exploit instability created by the Vietnam War to overthrow a weak U.S.-backed government in Cambodia in 1975. Inspired by the forced collectivization policies of Chinese leader Mao Tse Tung, Pol Pot then embarked on a bloody restructuring of society aimed at his vision of a utopian, agrarian state. As many as two million people were executed or died of disease, starvation or overwork in the regime's "Killing Fields".

In 1979, Vietnam invaded Cambodia and pushed Pol Pot and his remaining followers into the jungle, where they fought an unsuccessful insurgency for the next two decades. Pol Pot died in 1998 under suspicious circumstances.

In the past, both Nuon Chea and Khieu Samphan have maintained that they believed at the time they were acting in the best interests of the Cambodian people and that they were unaware of the full extent of the killings.